Meaning: Pay attention to the minutest detail.
Also: Mind your p's and q's
Matthew 5:18
For verily I say unto you, Till heaven and earth pass, one jot or one tittle shall in no wise pass from the law, till all be fulfilled.
Luke 16:17
And it is easier for heaven and earth to pass, than one tittle of the law to fail.
"They have so narrowlye loked on my transplatyon, that there is not so much as one I therin if it lacke a thrle over his hed, but they have noted it."
William Tyndale, The Obedience of a Christian Man (1540)